Bows have been used for many years as a weapon for hunting and target shooting. More advanced bows include cams that increase the mechanical advantage associated with the draw of the bowstring. The cams are configured to yield a decrease in draw force near full draw. Such cams preferably use power cables that load the bow limbs. Power cables can also be used to synchronize rotation of the cams, such as disclosed in U.S. Pat. No. 7,305,979 (Yehle).
With conventional bows and crossbows the draw string is typically pulled away from the generally concave area between the limbs and away from the riser and limbs. This design limits the power stroke for bows and crossbows.
In order to increase the power stroke, the draw string can be positioned on the down-range side of the string guides so that the draw string unrolls between the string guides toward the user as the bow is drawn, such as illustrated in U.S. Pat. No. 7,836,871 (Kempf) and U.S. Pat. No. 7,328,693 (Kempf). One drawback of this configuration is that the power cables can limit the rotation of the cams to about 270 degrees. In order to increase the length of the power stroke, the diameter of the pulleys needs to be increased. Increasing the size of the pulleys results in a larger and less usable bow.
FIGS. 1-3 illustrate a string guide system for a bow that includes power cables 20A, 20B (“20”) attached to respective string guides 22A, 22B (“22”) at first attachment points 24A, 24B (“24”). The second ends 26A, 26B (“26”) of the power cables 20 are attached to the axles 28A, 28B (“28”) of the opposite string guides 22. Draw string 30 engages down-range edges 46A, 46B of string guides 22 and is attached at draw string attachment points 44A, 44B (“44”)
As the draw string 30 is moved from released configuration 32 of FIG. 1 to drawn configuration 34 of FIGS. 2 and 3, the string guides 22 counter-rotate toward each other about 270 degrees. The draw string 30 unwinds between the string guides 22 from opposing cam journals 48A, 48B (“48”) in what is referred to as a reverse draw configuration. As the first attachment points 24 rotate in direction 36, the power cables 20 are wrapped around respective power cable take-up journal of the string guides 22, which in turn bends the limbs toward each other to store the energy needed for the bow to fire the arrow.
Further rotation of the string guides 22 in the direction 36 causes the power cables 20 to contact the power cable take-up journal, stopping rotation of the cam. The first attachment points 24 may also contact the power cables 20 at the locations 38A, 38B (“38”), preventing further rotation in the direction 36. As a result, rotation of the string guides 22 is limited to about 270 degrees, reducing the length 40 of the power stroke.
Various trigger systems are used to retain the draw string 30 in the drawn configuration, such as disclosed in U.S. Pat. No. 7,174,884 (Kempf); U.S. Pat. No. 7,770,567 (Yehle); and U.S. Pat. No. 8,240,299 (Kronengold). Due to the high forces generated by a crossbow, firm engagement is required between the seer and the trigger assembly. These high pressures combined with the solid engagement of the seer with the trigger assembly often results in an undesirably hard and rough trigger pull. Crossbows also require a system to prevent inadvertent dry firing. It is therefore desirable to provide a string control system for a crossbow that provides for a lighter, smoother trigger pull in combination with an anti-dry fire mechanism.